IEEE1547 DG protection requirements

[trat1208]

All,

I was wondering if anyone knew / understood why IEEE1547 is so specific about requiring phase-to-phase voltage detection for 27/59/81 tripping? (It of course gives the caveat that phase-to-neutral voltage can be used if the utility and DG are connected by a wye-wye grounded or single phase transformer). Is there any technical reason why they recommend using phase-to-phase voltage?

At my site (campus for a large company), we have two utility 34kV services connected to two 34-13.8kV transformers. On the 13.8kV busses, there are several distribution feeders as well as a 10MW combustion turbine generator that is impedance grounded (400A GF available). We currently have the 27/59/81 protection installed per IEEE1547, but how it works is a confusing mess due to some oddities of our infrastructure and probably poor initial planning when it was installed. We were planning to do a relay upgrade on the two 34kV lines (go to line current diff between us and the utility, existing protection on our end is 67 and 51 on their side) and was looking to simplify by putting the 27/59/81 protection in the line protection relays (in my mind this is where it should logically belong). However, the line current diff relay that was selected only offers a single pair of 27 and 59 phase-to-phase elements and a single pair of 27 and 59 phase-to-neutral elements. (This was an economic decision, the relay meets our needs as far as the line current diff protection goes and the ability to put the 27/59/81 protection in there was just an additional benefit if feasible, a line current diff relay that had the multiple phase-to-phase elements was going to cost several thousand dollars more each).

I really don't see any real difference between using the ph-ph and ph-n voltage for this purpose. My understanding of the IEEE1547 voltage-settings are that they're more there to protect the utility than to provide any real protection for the customer's DG. I.e. they ensure that a customer DG's trips off quickly when there is a utility fault so the DG won't be holding utility equipment energized and potentially interfering with reclosing or substation autotransfer operations. Also, the use of ph-ph voltage seems odd because typically the utility wants you to use wye-wye pots to supply these relays.

If anyone has any insight on this please let me know. Thanks.

 

[cranky108]

The phase to phase requirements are because of the likely use of delta to wye transformers used at the customer site. A phase to ground voltage on the secondary of a delta wye transformer would pickup phase to ground and phase events, while a phase to phase on the secondary is more of a three phase event detector.

For faults feeding circuits with frequency relays, there is a concern that the shifting of the zero crossing can operate the frequency relay, if the time delay is very short.

 

[trat1208]

Thanks cranky for the response. That clears up some things.